1. Field of the Invention
The present invention relates to a method of depositing a film and a sputtering apparatus for depositing a metal film or a metal alloy film (a compound of a metal film) by sputtering.
2. Description of the Related Art
A metal film on a semiconductor substrate used for LSI has mainly been deposited by sputtering. The reason why is that a good quality, uniform metal film low in impurity content can be deposited on the substrate surface at high speed in a stable manner without causing any damages thereto.
In a semiconductor device such as LSI or the like, a through-hole (or contact hole) is formed in an interlayer insulating film on a semiconductor substrate to connect a semiconductor element and wiring. The aspect ratio calculated as a ratio of "the depth of a through-hole/the diameter of a through-hole" has been increasing with the recent trend toward higher integration of LSI's. Therefore, it is difficult to cover, with high accuracy, the inner wall surface of a through-hole with a metal film, if a conventional sputtering technique is employed.
Therefore, a collimated sputtering technique and a long throw sputtering technique have been proposed as methods for improving the covering ability of a metal film formed on the bottom of a through hole in the case of a high aspect ratio. These methods have in common that the ratio of particles proceeding along a direction substantially perpendicular to the surface of a substrate is increased by sputtering. These sputtering methods will below described in a concrete manner.
The collimated sputtering technique is disclosed, for example, in the publication of Unexamined Japanese Patent Application No. Hei 1-116070, in which a target and a substrate are placed in a chamber and a collimating plate having a number of holes is located between the target and the substrate. Only particles proceeding in a direction substantially perpendicular to the substrate among particles, which proceed toward the substrate pass through the collimating plate and the others are caught by the collimating plate.
The long throw sputtering technique is described, for example, in an article authored by T. KIYOTA, et al. entitled "FILLING TECHNOLOGY FOR CONTACT HOLES WITH A HIGH ASPECT RATIO USING SPUTTERING WITHOUT A COLLIMATOR" in VLVAC, PP. 225-230, 1994. FIG. 1 is a typical view showing a sputtering apparatus when a metal film is deposited on a surface of a substrate by a long throw sputtering technique. A substrate holder 29 is placed in a chamber 22 and a semiconductor substrate 28, for example, of 8 inches in diameter is placed on the substrate holder 29. A target 25 is located above the substrate 28 and a cathode electrode 26 is placed in a manner such that the electrode 26 contacts the upper surface of the target 25. The cathode electrode 26 is connected to a direct current electric source 27 located outside the chamber 22. A gas inlet port 24, through which Ar gas is introduced into the chamber 22, is formed at the side wall thereof. A vacuum exhaust port 23, through which a gas within the chamber 22 is discharged, is formed at the bottom wall of the chamber 22.
A diameter of the target 25 is, for example, 300 mm and a distance between the target 25 and substrate 28 is, for example, 300 mm. A magnet (not shown) is placed near the rear side of the target 25 (near the side of the cathode electrode 26) so that a area, from which the most particles are released, is an area of about 150 mm in diameter in the target.
In the thus structured sputtering apparatus, the distance between the target 25 and substrate 28 is made longer than that of a conventional sputtering technique and sputtering is performed in a lower pressure, and thereby, particles can reach the substrate 28 without being scattered by molecules of a spattering gas.
FIGS. 2A and 2B are sectional views showing covering conditions of Ti films, when the Ti films each are formed on the surface of a substrate by a long throw sputtering technique. A silicon oxide film 42 is formed on the substrate 28 and through holes 42a and 42b are formed in the silicon oxide film 42. It should be noted that, the through hole 42a shown in FIG. 2A is formed at the vicinity of the central portion of the substrate 28, and the through hole 42b shown in FIG. 2B is formed at the peripheral portion of the substrate 28. If a Ti film is formed on the inner wall surface of the through holes 42a, 42b and the upper surface of the silicon oxide film 42 using a sputtering apparatus shown in FIG. 1, particles are released throughout the inner space of the through hole 42a in a uniform manner by sputtering. Therefore, as shown FIG. 2A, a uniform Ti film 41 is formed on the inner wall surface (on the side surface and bottom surface) of the through-hole 42a.
On the other hand, in the peripheral portion of the substrate 28, the number of particles released from the peripheral region of the target 25 by sputtering is smaller than that from the central portion of the target 25. Accordingly, in a through hole 42b formed in the oxide silicon film 42, the Ti film formed on the inner wall surface has a larger thickness in a portion nearer the periphery of the substrate 28 than that in a portion nearer the center thereof. In such a manner, even with use of the sputtering apparatus shown in FIG. 1, the Ti film formed on the inner wall surface of a through hole 42b formed at the peripheral portion of the substrate 28 has a non-uniform film thickness thereacross.
A sputtering apparatus is disclosed in the publication of Unexamined Japanese Patent Application No. Sho 63-162862, in which uniformity (step coverage) between a metal film formed on the inner wall surface of a through hole formed in an interlayer insulating film at the peripheral portion of a substrate and a metal film formed on the surface of the interlayer insulating film is improved. With the sputtering apparatus, a method in which either a target or a substrate is inclined, slid or rotated, or in combination, is employable to form a metal film on the inner wall surface of a through hole in a uniform manner.
A method or an apparatus to form a metal film on a substrate, in which either a target or a substrate is placed in an inclined condition relative to each other, is described in the publications of Unexamined Japanese Patent Application No. Sho 62-70568, Unexamined Japanese Patent Application No. Hei 4-311842, Unexamined Japanese Patent Application No. Hei 5-78831 and Unexamined Japanese Patent Application No. Hei 6-81145.
A method is disclosed in U.S. Pat. No. 4,664,935, in which sputtering is performed while a substrate is inclined relative to a target at 0 to 45.degree. therebetween and the substrate is rotated in order to improve uniformity in thickness, step coverage and surface form of a metal film formed by sputtering. In this method, the inclination of a substrate relative to a target is adjusted at 10 to 45.degree. therebetween in order to prevent a thicker metal film from being formed on the top portion of a through hole and the through hole from being partly closed (self shadowing).
When a metal film is formed on a surface of a substrate using one of the above mentioned conventional methods or apparatuses, however, there arise the following problems, one of which is that the covering rate of a metal film formed on the bottom surface of the through hole is low, and the other is that the utilization efficiency (or the maximum thickness of a metal film which can be formed with one target) of a target is low.